The present invention relates to a semiconductor memory device, and more particularly to a method and a circuit for repairing a defect by substituting a redundant memory cell for a defective memory cell in the semiconductor memory device.
In general, as semiconductor elements become small and highly integrated, there arise difficulties in fabricating them, which reduce the yield rate of the semiconductor memory devices. Reduction of the yield rate caused by the defective memory cell functions as a significant problem in the manufacturing of semiconductor memory devices. Therefore, in order to overcome this problem redundant cell are widely used. Thus, the yield rate is improved by substituting a redundant memory cell fabricated as a spare memory cell for a defective memory cell to repair the defect. It is necessary, however, store the position, i.e., the address, of the defective memory cell when substituting the redundant memory cell for the defective memory cell. Generally, there are two methods of storing the address, one of which is a fuse blowing method and the other is to use an electrically erasable and programmable nonvolatile memory cell as the storage device of the defective address. With the fuse blowing method, the defective address is stored by performing a fuse blowing operation through the flow of current or irradiation of a laser beam toward the fuse elements to store the defective address.
The laser fuse blowing method can be used in only a wafer state previous to a package process, i.e., a fuse exposure state, but it can not be used after the package process is complete.
On the other hand, when nonvolatile memory cell is used as the defective address storing device, it is possible to store the address even in the packaged state. However, since the memory device itself is a memory cell, if the specific state, e.g., high temperature and high voltage state of this defective address information, is not kept, malfunctions may arise in the semiconductor memory device. Therefore, the reliability of memory devices which use the nonvolatile memory cell method is less than the fuse blowing method. Thus, an electrical fuse blowing method is widely used for storing the address of defective memory cells. Such an electrical fuse blowing method is disclosed at pages 128.about.129 of a paper in "IEEE INTERNATIONAL SOLID STATE CIRCUITS CONFERENCE DIGEST OF TECHNICAL PAPERS" published on 1989.
However, in the conventional electrical fuse blowing method, since a number of fuses are simultaneously blown to store the defective address, it is necessary to provide enough current to blow many fuses. Further, due to the need for more current an added pad or pressure pin is needed, which also causes problems, since such a voltage application pad additionally formed in the wafer state adversely influences the integration of the elements. Moreover, in order to provide sufficient current for fuse blowing in the package state, since the high voltage is applied to the internal of the memory device through the added input/output pin, there arises the other problem in that the spare pin is necessary.